The removal of oxides of nitrogen from oxygen rich streams is currently performed at a commercial level in utility boiler exhausts using selective catalytic reduction (SCR). In this process, an exhaust stream is passed over a catalyst, along with a reductant (ammonia). Through a chemical reaction in which ammonia reduces nitrogen oxides (NO.sub.x) adsorbed onto the catalyst surface, the latter species (NO.sub.x) are removed from the exhaust stream. The process thus requires the availability of an on-site source of ammonia or other reductant. This adds capital cost in the form of an ammonia plant, as well as costs for fixturing and plumbing. The expense of operating such facilities must also be added to normal operating cost.
Conditions encountered by the catalyst(s) can be summarized as follows: temperature of 150-400.degree. C., oxygen content of 3-5%, NO content of 100-500 ppm for coal and 50-150 ppm for natural gas fired boilers, SO.sub.2 levels of 400-3000 ppm in exhausts of coal fired systems and negligible levels from natural gas fired boilers. Additionally, CO.sub.2 (10-20%) and H.sub.2 O (10-20%) are invariably present in exhaust streams. The exhaust streams will also typically contain incompletely oxidized species such as carbon monoxide, unburned hydrocarbon, or carbon particulates which, with the catalysts of the present invention, can be used to effect greater conversion of nitrogen oxides. Consequently, catalysts must be designed that will accommodate the presence of both this incidental reductant and excess oxygen. The use of reductants (ammonia) in addition to those from fuel is undesirable because of increased storage and operating cost and the opportunity for slip (emission of reductant). Additional concerns arise from the effect of fuel bound poisons such as sulfur dioxide, water, and carbon dioxide on the activity of a catalyst. High concentrations of water may chemically destabilize materials, thereby resulting in another pathway for catalyst deactivation. Fly ash can coat catalysts and introduce metallic poisons onto the surface of the catalysts.
The problem posed by catalytic deNO.sub.x in oxygen rich streams is that the catalyst is poisoned by adsorbed oxygen, which is typically present in concentrations from one to three orders of magnitude higher than that of the nitrogen oxides, resulting in a mass action effect (i.e. the extent of catalyst surface coverage by oxygen increases with gas stream oxygen content). The problem with oxide forming metals is that the metal surface rapidly forms a thin layer of adsorbed oxygen or metal oxide, blocking access to the surface of the molecule targeted for removal.
The direct catalytic conversion of the nitric oxide without a reducing agent has been previously investigated. Tabata (Kenji Tabata, Journal of Materials Science Letters, 7, 147(1988)) reported on the activity of the material Ba.sub.2 YCu.sub.3 O.sub.7-.delta.. It was found that the material could remove approximately 18% of a stream 51 ppm in NO and 8.1% in oxygen at approximately 300.degree. C. and 15,500 h.sup.-1.
Similar to the material reported by Tabata are the catalysts reported by Mori, et al, Toshiyuki Mori, Hiroshi Yamamura, Hiroyuki Ogino, Hidehiko Kobayashi, and Takashi Mitamura, Journal of the American Ceramic Society, 77, 2771(1994). The materials Ba.sub.2 In.sub.2 O.sub.5, Ba.sub.3 Y.sub.4 O.sub.9, and BaLa.sub.2 O.sub.4, which possess the brownmillerite structure, demonstrated deNO.sub.x activities which increased with a decreased temperature of orderd-disorder transition. In the absence of added oxygen, up to 45% of 1300 ppm NO was removed at 500.degree. C. and a space velocity of 2500 h.sup.-1. The authors indicated that these materials are sensitive to water, becoming less active in its presence.
Shin, et al. (Shigemitsu Shin, Yuko Hatakeyama, Kiyoshi Ogawa, and Kin'ya Shimomura, Materials Research Bulletin, 14, 133(1979)) evaluated the materials Ca.sub.2 Fe.sub.2 O.sub.5 and Sr.sub.2 Fe.sub.2 O.sub.5 for activity toward direct decomposition of NO.sub.x. The material Ca.sub.2 Fe.sub.2 O.sub.5 was inactive, but Sr.sub.2 Fe.sub.2 O.sub.5 demonstrated activity for the decomposition of 3% NO in helium. Essentially 100% of NO was decomposed at 900.degree. C. but only 300 h.sup.-1. Neither of these conditions suggest possible employment in utility applications.
Li and Hall (Yuejin Lin and W. Keith Hall, Journal of Physical Chemistry, 94, 6145 (1990)) reported on the activity of the zeolite catalyst Cu-ZSM-5 under conditions absent reagent (e.g., hydrocarbon). However, experiments were performed at impractically high levels of NO (4%) and absent oxygen. At very low space velocities (1800 h.sup.-1), the catalyst converted over 90% of the NO in the gas stream at 500.degree. C. The addition of 10% O.sub.2 to the reaction stream resulted in a decrease in NO conversion from 97% to 80%, pointing to some oxygen poisoning.
Another catalytic approach to decomposing nitrogen oxides is that of Lunsford, et al. (Shuibo Xie, Gerhard Mestl, Michael P. Rosynek, and Jack H. Lunsford, Journal of the American Chemical Society, 119, 10186 (1997)). In this case, MgO supported BaO was the catalyst. The material was found to exhibit a maximum in the decomposition of 1% NO, absent oxygen, at 630.degree. C. The addition of 0.5% oxygen to the gas stream resulted in almost complete loss of activity at that temperature. However, most of the activity was recovered when the temperature was increased to 660.degree. C. When only 1.2% CO.sub.2 was added to the gas stream, the catalyst's activity was decreased by almost a factor of ten.
In an article due to Wu, et al. (Zhen Zhao, Xiangguang Yang, and Yue Wo, Applied Catalysis B: Environmental, 8), the perovskite materials LaNiO.sub.3, La.sub.0.1 Sr.sub.0.9 NiO.sub.3, La.sub.2 NiO.sub.4, and LaSrNiO.sub.4 were evaluated for NO uptake, temperatures of desorption, and activity toward NO decomposition. It was found that catalyst activity increased with increasing Sr content, indicating that catalyst activity was proportional to the concentration of oxygen ion vacancies in the catalyst. The most active of these catalysts achieved its maximum activity (95% NO conversion) at 900.degree. C., a far greater temperature than the maxima of the specific examples of the present invention which show maximum activity restricts the materials reported by Wu from finding use in post-combustion environments such as utility boiler exhausts.
The catalysts of the present invention possess a number of attributes which distinguish them from the prior art and which provide them with distinct advantages over the prior art.
First, the materials of the present invention function with practical activity under conditions with large excesses of oxygen present, and without the intentional addition of a reagent (ammonia, urea, or hydrocarbon). This cannot be said of any of the prior art. This feature promises to make the catalysts of this invention much more economical than state-of-the-art materials, and to permit application in the mitigation of NO.sub.x in the exhaust streams of utility boilers.
Second, the materials are easily synthesized and loaded onto supports in a variety of ways. This is particularly important when comparing the catalysts to zeolite materials. In the latter case, the material must be hydrothermally synthesized, i.e., from aqueous solutions under conditions of elevated temperature and pressure. This imposes severe constraints on the use of these materials in exhaust emissions where operation with minimum pressure drop is critical, since the fabrication of zeolite coated monoliths may be prohibitively expensive. For example, the use of catalyst pellets or powders in the exhausts of gas turbine engines may not be feasible. This discussion of zeolites immediately prompts the third distinguishing feature of the materials; their hydrothermal stability. Zeolites, being prepared from aqueous solutions under conditions of high temperature and pressure, also tend to be unstable under conditions in which the catalyst is steamed at high temperature. This is not true of the catalysts of the present invention.
A fourth feature of the present catalysts is their tolerance toward major gas stream constituents such as carbon dioxide and water. While simple base catalysts such as that of Lunsford, et al. demonstrate some direct decomposition activity, they are rapidly poisoned in the presence of species such as carbon dioxide, losing much of their initial activity. The present catalysts do not exhibit such intolerance.
The fifth feature of the present catalysts is the ability to obtain them in specific compositional forms incorporating catalyst functions of basicity, structurally stabilizing Lewis acid cations, and active transition-metal cations which give rise to the desirable properties of the catalysts. Thus, while Shin, et al. and Mori, et al. report brownmillerite materials, none of these materials possess practical activity or poison tolerance. It is the capability of manipulating composition and structure toward utility in practical applications that is a significant property of the materials of the present invention, not the specific nature.